Industrial applications such as robotics, automated machine tools and precise positioning systems are generally implemented with DC-servo motors as power sources. DC motors are commonly used for applications requiring characteristics such as fast response and good low speed control because the torque developed by a DC motor is proportional to the armature current. The armature current of a DC motor is easily controlled using current feedback. Although the AC induction motor enjoys a number of advantages such as higher output per unit mass over the DC motor, the fact that rotor flux is induced rather than directly controlled has traditionally disqualified the AC induction motor from many high performance, high precision industrial applications.
The recent development of field oriented control has now made fast torque control of induction motors achievable. Analogous to the DC machine, the torque control of an AC motor will be achieved through current control, however, in the AC case this must also include the phase as well as the amplitude.
In the DC motor the commutator and the brushes fix the orientation of the field flux and the armature mmf. However, in the AC machine, the orientation of the field flux and armature mmf must be controlled externally. If this orientation is not controlled in the AC motor then the angles between the various fields vary with loads and transients making for an oscillatory dynamic response. With the orientation between the fields controlled externally in the AC motor, current feedback can be applied to allow torque control.
Various approaches have been made to qualify the induction motor for high performance applications using various concepts of field oriented control. One approach involves the measurement of rotor flux and the determination of stator terminal excitation values needed to produce desired torque or speed conditions.
As a practical matter it is impossible to accurately measure rotor flux vector position and magnitude in an induction motor. It is known to place flux detectors in induction motors and to utilize such devices in control systems. However, complex adjustable filters are required, since strong slot harmonics, the frequency of which is speed dependent, are superimposed upon the fundamental signal. Generally, the accuracy of a torque signal computer from this information is likely to be poor, since torque is an integral quantity which is difficult to measure on the basis of local field measurements.